Graphite material, carbon material for battery electrode, and battery

ABSTRACT

A carbon raw material such as a green coke in which loss on heat when it is heated from 300 to 1200° C. under an inert atmosphere is no less than 5% by mass and no more than 20% by mass is pulverized and then the pulverized carbon raw material is graphitized to obtain a graphite material suitable for a carbon material for anode in a lithium-ion secondary battery or the like that enables to make electrodes having a high-energy density and a large-current load characteristic since it has a small specific surface area and a small average particle diameter while maintaining high beginning efficiency and a high discharge capacity in the first round of charging and discharging. And an electrode for batteries are obtained using the graphite material.

TECHNICAL FIELD

This application claims priority based on Japanese Application Nos.2005-350138, 2005-365878 and 2005-368727 filed on Dec. 5, 2005, Dec. 20,2005 and Dec. 21, 2005, respectively, the entire contents of which arehereby incorporated by reference.

The present invention relates to a graphite material, a carbon materialfor battery electrodes and a battery. More particularly, the inventionrelates to a graphite material and a carbon material for batteryelectrodes that are suitable as electrode materials for nonaqueouselectrolyte solution secondary batteries, and also relates to asecondary battery having an excellent charge-discharge cyclecharacteristics and an excellent large-current load characteristics.

BACKGROUND ART

Lithium secondary batteries are mainly used as power supplies inportable devices and the like. Portable devices and the like have avariety of capabilities and thus consume a large amount of electricpower. For this reason, lithium secondary batteries are required toincrease battery capacities and to enhance charge-discharge cyclecharacteristics. High-power, high-capacity secondary batteries for usein electric tools such as an electric drill, hybrid cars and the likeare increasingly required. Conventionally, lead-acid secondarybatteries, nickel-cadmium secondary batteries and nickel-metal-hydridesecondary batteries are mainly used in these fields. Compact,lightweight and high-energy density lithium-ion secondary batteries,however, are highly expected to be used, and thus lithium-ion secondarybatteries having excellent large-current load characteristics aredesired.

In general, in the lithium secondary batteries, lithium salt such aslithium cobaltate is used as a cathode active material, and carbonaceousmaterial such as graphite is used as an anode active material.

Mesocarbon spherules are widely used as graphite serving as a anodeactive material. The production process of the mesocarbon spherules is,however, complicated, and this makes it extremely difficult to reducecost of the mesocarbon spherules.

In Graphite, there are natural graphite and artificial graphite. Thenatural graphite is available at low cost. However, it is shaped likescales. Thus, when the natural graphite and binder are mixed into apaste and the paste is then applied to a collector, the natural graphiteis oriented in only one direction. When charging is performed using suchan electrode, the electrode expands in only one direction, and thiscauses degraded performance of the electrode. Although it is suggestedthat natural graphite be spherically granulated, the sphericallygranulated natural graphite collapses to become oriented in the samedirection by being pressed when electrodes are produced. In addition,since the surface of the natural graphite is active, a large amount ofgas is generated in the first charging, resulting in reduced beginningefficiency and a poor cycle characteristic.

Artificial graphite typified by graphitized products from petroleum oil,coal pitch, coke and the like is available at relatively low cost. It ishigh in strength and resistant to collapse. However, needle coke that iseasily crystallized is likely to form a scale-like shape to becomeoriented in the same direction. Non-needle coke is likely to formsubstantially spherical particles, but it often has a slightly lowdischarge capacity and poor beginning efficiency.

Under these situations, instead of mesocarbon spherules, variousinexpensive graphite materials for battery electrodes are beingresearched. Patent document 1 discloses a carbon material for an anodein a lithium secondary battery, namely, graphitized carbon powderprepared by subjecting carbon powder made of pitch in the presence of aboron compound to heat treatment, characterized in that the coefficientof thermal expansion (CTE) of the carbon powder, the interplanar spacing(d₀₀₂) of graphite plane as measured by X-ray diffraction, the length(Lc) of a crystallite in the direction of the C-axis and the ratio(R=I₁₃₆₀/I₁₅₈₀) of the strength of 1360 cm⁻¹ band to the strength of1580 cm⁻¹ band as measured by Raman spectroscopy using an argon laserare CTE≦3.0×10⁻⁶° C.⁻¹, d₀₀₂≦0.337 nm, Lc≧40 nm and R≧0.6, respectively.

Patent document 2 discloses a carbon material for an anode in a lithiumsecondary battery, namely, graphitized carbon powder obtained bygraphitizing green coke powder produced from at least one of coke rawmaterials of petroleum-derived or coal-derived heavy oils after it isheated and oxidized under an atmosphere of oxidized gas, characterizedin that, the interplanar spacing (d₀₀₂) of graphite planes of the carbonpowder as measured by wide angle X-ray diffraction, the length (Lc) of acrystallite in the direction of the C-axis, the coefficient of thermalexpansion (CTE) and the ratio (R=I₁₃₆₀/I₁₅₈₀) of the strength of a peakin the vicinity of 1360 cm⁻¹ to the strength of a peak in the vicinityof 1580 cm⁻¹ as measured by Raman spectroscopy using an argon laser ared₀₀₂≦0.337 nm, Lc≧30 nm, CTE≧3.0×10⁻⁶° C.⁻¹ and R≧0.3, respectively.

In patent document 3, the assignee of the present invention discloses acarbon material for a lithium battery, the carbon material beingcomposed of graphite powder that is obtained by pulverizing andgraphitizing calcined coke and is characterized in that the specificsurface area is no more than 3 m²/g, the aspect ratio is no more than 6and the tapping bulk density is no less than 0.8 g/cm³.

-   Patent document 1: JP-A-H08-031422-   Patent document 2: JP-A-H10-326611-   Patent document 3: WO 00/22687

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

With the graphite materials proposed in patent document 1 or 2, however,it is impossible to obtain sufficient beginning efficiency and asufficient discharge capacity. With the graphite material proposed inpatent documents 3, it is possible to obtain a high discharge capacity,an excellent cycle characteristic and an excellent charge-dischargeefficiency. The graphite materials proposed in patent documents 1 to 3have a great tendency to have a tap density slightly lower than themesocarbon spherules. This is probably due to the shape of particles andmore specifically, for example, the aspect ratio. The low tap densityprevents the electrode density from increasing, with the result that thebattery capacity decreases. In a case where the same particle sizedistribution is achieved, as compared with the mesocarbon spherules, thegraphite materials inevitably tend to have a larger specific surfacearea. An increase in the specific surface area causes an increase in theamount of binder used, and this increases the proportion of the binderin the electrode. Since the binder is not involved in charging anddischarging, the battery capacity decreases.

As will be understood from the above description, it is quite difficultto obtain sufficient performance using the anode material produced bycrushing coke, instead of using the mesocarbon spherules.

The inventors of the present invention have found that it is possible toreduce the diffusion time of lithium in graphite and thus improve thecurrent load characteristic of anode material by decreasing the diameterof graphite material and more specifically by decreasing D50 from about15 μm, which is typical, to about 2-9 μm.

However, when the particle size was reduced to such a particle size withthe methods disclosed in patent document 1, 2 or 3, the specific surfacearea was found to be 10 m²/g or more in most cases. Although the largespecific area advantageously increases the number of the sites throughwhich lithium ions enter between graphite layers, chemically activeportions are increased and thus the amount of SEI (solid electrolyteinterface) or the like generated is increased. This leads tosignificantly degraded charge-discharge efficiency in the first round ofcharging and discharging. Moreover, when the specific surface area isincreased, the amount of binder used is increased, and thus the amountof anode material in an electrode is reduced.

In order to obtain a battery with priority given to the large-currentload characteristic, it is necessary to reduce the internal resistanceof the battery. One way to achieve this is to reduce the thickness ofapplied material on an electrode. It is thought that when SBR(styrene-butadiene rubber) based binders are used, graphite materialsthat electrodes can be made of are of a specific surface area of about 4to 6 m²/g in terms of viscosity of the binders. It is also thought thatwhen PVDF (polyvinylidene-difluoride) based binders are used, graphitematerials that electrodes can be made of are of a specific surface areaof 2 to 3 m²/g or less. It is extremely difficult to produce graphitematerials having such a particle size and a specific surface area, withthe exception of some expensive mesocarbon.

It is an object of the present invention to provide, at a low cost, agraphite material suitable for an electrode active material of alithium-ion secondary battery and characterized in that its beginningefficiency in the first round of charging and discharging is much higherthan that of conventional graphite materials, its charge capacity ishigh and the particles of the graphite material have a low aspect ratio.

It is another object of the present invention to provide, at a low cost,a graphite material that is suitable for an anode carbon material of alithium-ion secondary battery or the like, capable to make electrodeshaving a high-energy density thereof since the graphite material has ahigh tap density and the particles of the graphite material have a lowspecific area and a low aspect ratio, while its beginning efficiency anddischarge capacity in the first round of charging and discharging arekept equal to or higher than those of conventional graphite materials.

It is yet another object of the present invention to provide, at a lowcost, a graphite material that is suitable for the anode carbon materialof a lithium-ion secondary battery or the like, capable to makeelectrodes having a high-energy density and a large-current loadcharacteristic thereof since the graphite material has a small specificsurface area and a small average particle diameter, while its beginningefficiency and discharge capacity in the first round of charging anddischarging are kept equal to or higher than those of conventionalgraphite materials.

Means for Solving the Problem

When petroleum coke is produced, the temperature of a coker is generallyabout 500° C., and the produced green coke still contains moisture andvolatile components. Thereafter, in order for the volatile components tobe removed, in general, the green coke is calcined at about 1200° C.When the calcined coke is pulverized, however, projections anddepressions are formed on the surface of the calcined coke. Thus, thegranular calcined coke having a high aspect ratio is obtained. Even whenthis granular calcined coke is graphitized, the projections anddepressions on the surface are not sufficiently smoothened, and thespecific surface area is not reduced as expected.

The inventors of the present invention found the following. Instead ofcommon calcined coke, green coke still containing volatile components ispulverized and is then graphitized. In this way, it is possible toreduce the aspect ratio, the size of projections and depressions on thesurface of the particles and the specific surface area aftergraphitization.

After detailed consideration, the inventors of the present inventionfurther found the following. A carbon raw material in which the loss onheat when the carbon raw material is heated from 300 to 1200° C. underan inert atmosphere ranges from 5% by mass to 20% by mass is pulverizedwithout being calcined and is then subjected to heat treatment(graphitization) under specifically given conditions. The graphitematerial obtained in such an inexpensive and simple method hassubstantially spherical particles. When the graphite material is used aselectrode material, a high capacity, an excellent cycle characteristicand an extremely low irreversible capacity are achieved.

With the method described above, the inventors of the present inventionalso found the following. A graphite material is obtained in which theprimary particles have an aspect ratio of 1.00 to 1.32 and in which nocoating layer is substantially included in the surface of the particles,or substantially single-composition particles having an isotropiccrystal structure are comprised. Moreover, the inventors of the presentinvention found that when the graphite material is used as electrodematerial, a high capacity, an excellent cycle characteristic and anextremely low irreversible capacity are achieved.

With the method described above, the inventors of the present inventionstill further found the following. A graphite material is obtained inwhich D50% is 2 to 9 μm in particle diameter distribution based onvolume as measured by laser diffraction, and the specific surface areais 2 to 6 m²/g, and in which no coating layer is substantially includedin the surface of the particles or substantially single-compositionparticles having an isotropic crystal structure are comprised. Moreover,the inventors of the present invention found that when the graphitematerial is used as electrode material, a high capacity, an excellentcycle characteristic and an extremely low irreversible capacity areachieved.

After further consideration based on these findings, the presentinvention is completed.

The present invention includes the following aspects.

-   (1) A method for producing a graphite material, comprising the steps    of: pulverizing a carbon raw material in which loss on heat when the    carbon raw material is heated from 300 to 1200° C. under an inert    atmosphere is no less than 5% by mass and no more than 20% by mass;    and then graphitizing the pulverized carbon raw material.-   (2) The method for producing a graphite material according to (1),    where the carbon raw material is a petroleum-derived pitch coke or a    coal-derived pitch coke.-   (3) The method for producing a graphite material according to (1),    wherein the carbon raw material is a green coke.-   (4) The method for producing a graphite material according to any    one of (1) to (3), wherein the graphitization temperature is no less    than 3000° C. and no more than 3300° C.-   (5) The method for producing a graphite material according to any    one of (1) to (4), wherein the carbon raw material is a non-needle    coke.-   (6) The method for producing a graphite material according to any    one of (1) to (5), wherein the graphitization is performed in an    Acheson furnace.-   (7) A graphite material obtained by the producing method according    to any one of (1) to (6).-   (8) The graphite material according to (7), wherein a ratio    I_(D)/I_(G) (R value) of a peak strength (I_(D)) in a vicinity of    1360 cm⁻¹ to a peak strength (I_(G)) in a vicinity of 1580 cm⁻¹ as    measured by Raman spectroscopy is no less than 0.01 and no more than    0.2, and a coefficient of thermal expansion (CTE) at temperatures of    30 to 100° C. is no less than 4.0×10⁻⁶/° C. and no more than    5.0×10⁻⁶/° C.-   (9) The graphite material according to (7) or (8), wherein D50% is    10 to 25 μm in particle diameter distribution based on volume as    measured by laser diffraction.-   (10) The graphite material according to any one of (7) to (9),    wherein a loose bulk density is no less than 0.70 g/cm³, and a    powder density is no less than 1.0 g/cm³ and no more than 1.35 g/cm³    after tapping is performed 400 times.-   (11) The graphite material according to any one of (7) to (10),    wherein a specific surface area is 0.8 to 1.8 m²/g.-   (12) The graphite material according to any one of (7) to (11),    wherein an average interplanar spacing d₀₀₂ of (002) plane as    measured by X-ray diffraction is 0.3362 nm to 0.3370 nm.-   (13) The graphite material according to any one of (7) to (12),    wherein an aspect ratio determined from an optical microscope image    is no less than 1 and no more than 5.-   (14) A graphite material, wherein an aspect ratio of a primary    particle is 1.00 to 1.32, and no coating layer is substantially    included in a surface of the particle.-   (15) A graphite material, comprising substantially    single-composition particles having an isotropic crystal structure,    wherein an aspect ratio of a primary particle is 1.00 to 1.32.-   (16) The graphite material according to (14) or (15), wherein a    non-needle coke is used as a raw material.-   (17) The graphite material according to (16), wherein the non-needle    coke is a petroleum-derived pitch coke.-   (18) The graphite material according to any one of (14) to (17),    wherein a laser Raman R value is no less than 0.01 and no more than    0.2, and a CTE at temperatures of 30 to 100° C. is no less than    4.0×10⁻⁶/° C. and no more than 5.0×10⁻⁶/° C.-   (19) The graphite material according to any one of (14) to (18),    wherein D50% is 10 to 25 μm in particle diameter distribution based    on volume as measured by laser diffraction.-   (20) The graphite material according to any one of (14) to (19),    wherein d₀₀₂ is 0.3362 nm to 0.3370 nm.-   (21) The graphite material according to any one of (14) to (20),    wherein a specific surface area is 0.8 to 1.8 m²/g.-   (22) The graphite material according to any one of (14) to (21),    wherein a loose bulk density is no less than 0.7 g/cm³, and a powder    density is no less than 1.0 g/cm³ and no more than 1.35 g/cm³ after    tapping is performed 400 times.-   (23) A graphite material, wherein D50% is 2 to 9 μm in particle    diameter distribution based on volume as measured by laser    diffraction, a specific surface area is 2 to 6 m²/g and no coating    layer is substantially included in a surface of the particle.-   (24) A graphite material comprising substantially single-composition    particles having an isotropic crystal structure, wherein D50% is 2    to 9 μm in particle diameter distribution based on volume as    measured by laser diffraction, and a specific surface area is 2 to 6    m²/g.-   (25) The graphite material according to (23) or (24), wherein a    non-needle coke is used as a raw material.-   (26) The graphite material according to (25), wherein the non-needle    coke is a petroleum-derived pitch coke.-   (27) The graphite material according to any one of (23) to (26),    wherein a laser Raman R value is no less than 0.01 and no more than    0.2, and a CTE at temperatures of 30 to 100° C. is no less than    4.0×10⁻⁶/° C. and no more than 5.0×10⁻⁶/° C.-   (28) The graphite material according to any one of (23) to (27),    wherein d₀₀₂ is 0.3362 nm to 0.3370 nm.-   (29) The graphite material according to any one of (23) to (28),    wherein an aspect ratio of a primary particle is 1.00 to 1.32.-   (30) The graphite material according to any one of (23) to (29),    wherein a loose bulk density is no less than 0.4 g/cm³, and a powder    density is no less than 0.5 g/cm³ and no more than 1 g/cm³ after    tapping is performed 400 times.-   (31) A carbon material for battery electrodes, the carbon material    comprising the graphite material according to any one of (7) to    (30).-   (32) The carbon material for battery electrodes according to (31),    the carbon material further comprising a carbon fiber having a fiber    diameter of 2 to 1000 nm.-   (33) The carbon material for battery electrodes according to (32),    wherein 0.01 to 20 parts by mass of the carbon fiber are contained    with respect to 100 parts by mass of the graphite material.-   (34) The carbon material for battery electrodes according to (32) or    (33), wherein the carbon fiber has an aspect ratio of 10 to 15000.-   (35) The carbon material for battery electrodes according to any one    of (32) to (34), wherein the carbon fiber is a vapor grown carbon    fiber.-   (36) The carbon material for battery electrodes according to any one    of (32) to (35), wherein the carbon fiber is subjected to a heat    treatment at a temperature of 2000° C. or higher.-   (37) The carbon material for battery electrodes according to any one    of (32) to (36), wherein the carbon fiber has a hollow structure    inside the carbon fiber.-   (38) The carbon material for battery electrodes according to any one    of (32) to (37), wherein the carbon fiber comprises a branched    carbon fiber.-   (39) The carbon material for battery electrodes according to any one    of (32) to (38), wherein the carbon fiber is no more than 0.344 nm    in an average interplanar spacing d₀₀₂ of (002) plane as measured by    X-ray diffraction.-   (40) A paste for electrodes, the paste comprising the carbon    material for battery electrodes according to any one of (31) to (39)    and a binder.-   (41) An electrode composed of a molded piece of the paste for    electrodes according to (40).-   (42) A battery comprising the electrode according to (41) as a    component.-   (43) A secondary battery comprising the electrode according to (41)    as a component.-   (44) An electric tool comprising the battery according to any one    of (41) to (43) as a component.-   (45) A car comprising the battery according to any one of (41)    to (43) as a component.

Effects of the Invention

When the graphite materials according to the present invention are usedas carbon material for battery electrodes, high-energy density batteryelectrodes can be obtained while a high capacity, high coulombicefficiency and an excellent cycle characteristic are maintained.

The producing method according to the present invention is a method thatachieves increased economy, increased productivity and improved safety.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing a cross-sectional image of mesocarbon asobserved by transmission electron microscopy (TEM); and

FIG. 2 is a diagram showing a cross-sectional image of graphite materialaccording to the present invention as observed by TEM.

EXPLANATION OF REFERENCE SYMBOLS A Surface layer (coating layer) B Innerlayer C Boundary

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in detail.

(Graphite Material 1)

In a graphite material 1 according to a first embodiment of the presentinvention, the ratio I_(D)/I_(G) (R value) of a peak strength (I_(D)) inthe vicinity of 1360 cm⁻¹ to a peak strength (I_(G)) in the vicinity of1580 cm⁻¹ as measured by Raman spectroscopy is preferably no less than0.01 and no more than 0.2. When the R value is more than 0.2, a largenumber of highly activated edge portions of graphite material areexposed to the surface thereof. Thus, during charging and discharging, alarge number of side reactions occur. In contrast, when the R value isless than 0.01, barriers that prevent the entering and exiting oflithium are higher, and this causes the current load characteristic tobe degraded.

The laser Raman R value is measured using “NRS3100” made by JASCOCorporation under the conditions that the excitation wavelength is 532nm, the width of the entrance slit is 200 μm, the exposure period is 15seconds, the totaling is performed twice and the diffraction grating has600 bars per millimeter.

In the graphite material 1 in the present invention, the coefficient ofthermal expansion (CTE) at temperatures of 30 to 100° C. is preferablyno less than 4.0×10⁻⁶/° C. and no more than 5.0×10⁻⁶/° C. Thecoefficient of thermal expansion is utilized as an indicator that showsthe acicularity of coke. Although graphite materials with a coefficientof thermal expansion of lower than 4.0×10⁻⁶/° C. have high crystallinityof graphite and a high discharge capacity, the particles thereof aremore likely to form a plate-like shape. In contrast, although graphitematerials with a coefficient of thermal expansion of higher than5.0×10⁻⁶/° C. have a low aspect ratio, the crystals of graphite are notgrown, and a discharge capacity is low. The CTE of the graphite material1 is measured in the same manner as that of a carbon raw material, whichwill be described later.

In the graphite material 1 in the present invention, the averageinterplanar spacing d₀₀₂ of (002) plane as measured by X-ray diffractionis preferably 0.3362 nm to 0.3370 nm. The interplanar spacing d₀₀₂ canbe measured in a known manner by powder X-ray diffraction (XRD) (see the117th committee material 117-71-A-1 (1963) of Japan Society for thePromotion of Science by Inakichi Noda and Michio Inagaki, the 117thcommittee material 117-121-C-5 (1972) of Japan Society for the Promotionof Science by Michio Inagaki et al. and “carbon” No. 36, pp. 25-34, 1963by Michio Inagaki).

In the graphite material 1 in the present invention, the aspect ratio(the length of a long axis/the length of a short axis) is preferably nomore than 6, and more preferably is no less than 1 and no more than 5.The aspect ratio can be determined from optical microscope images. Theaspect ratio may simply be measured by image analysis using “FPIA3000”made by Sysmex Corporation.

In the graphite material 1 in the present invention, the specificsurface area (as measured by BET method) is preferably no more than 2m²/g, and more preferably 0.8 to 1.8 m²/g. When the graphite materialhas a specific surface area of more than 2 m²/g, the surface activity ofthe particles thereof is higher, and the coulombic efficiency may bedecreased due to decomposition of electrolytic solution or otherfactors.

Preferably, in the graphite material 1 in the present invention, theloose bulk density is no less than 0.70 g/cm³, and the powder density isno less than 1.0 g/cm³ and no more than 1.35 g/cm³ after tapping isperformed 400 times.

In the graphite material 1 in the present invention, D50% is preferably10 to 25 μm in particle diameter distribution based on volume asmeasured by laser diffraction.

(Graphite Material 2)

In a graphite material 2 according to a second embodiment of the presentinvention, the primary particles have an aspect ratio of 1.00 to 1.32and no coating layer is substantially included in the surface of theparticles. Or the graphite material 2 in the present invention comprisessubstantially single-composition particles having an isotropic crystalstructure, wherein the primary particles have an aspect ratio of 1.00 to1.32.

FIG. 1 shows a cross-sectional image of meso-phase carbon as observed bytransmission electron microscopy (TEM); FIG. 2 shows a cross-sectionalimage of the graphite material 2 in the present invention as observed byTEM.

As will be understood from FIG. 1, in the meso-phase carbon, its surfacelayer A has a white color (in a state where electron beams are morelikely to be transmitted), and its inner layer B has a gray color (in astate where electron beams are less likely to be transmitted). Aboundary C appears clearly between the surface layer and the innerlayer.

In contrast, as shown in FIG. 2, the graphite material 2 in the presentinvention has no boundary between the surface layer and the inner layerand has a gray color evenly from the surface layer to the inner layer.That is, in the graphite material in the present invention, there issubstantially no coating layer, or an isotropic crystal structure isformed and a single composition is substantially formed.

In the graphite material 2 in the present invention, the aspect ratio(the maximum length D_(max)/the vertical length relative to the maximumlength DN_(max) (D_(max)=the maximum length between any two points onthe perimeter of the image of a particle; DN_(max)=the shortest lengthof a line that, when the image is interposed between two lines parallelto the line of the maximum length, vertically connects the two lines) is1.0 to 1.32. The aspect ratio can be measured by image analysis using“FPIA3000” made by Sysmex Corporation. At least 3000 points, preferably30000 points or more, more preferably 50000 points or more are measured,and the average of the measured values is used.

In the graphite material 2 in the present invention, the specificsurface area (as measured by BET method) is preferably no more than 2m²/g, more preferably 0.8 to 1.8 m²/g. When the graphite material has aspecific surface area of more than 2 m²/g, a relatively large amount ofPVDF (polyvinylidene-fluoride) based binder or even SBR(styrene-butadiene rubber) based binder needs to be added. This leads tothe decreased mass of active material per unit electrode volume, withthe result that the battery capacity is decreased. Since the specificsurface area is increased, the surface activity of the particles becomeshigher, with the result that the coulombic efficiency may be decreaseddue to decomposition of electrolytic solution or other factors. Thegraphite material in the present invention has D50% of preferably 10 to25 μm in particle diameter distribution based on volume as measured bylaser diffraction.

In the graphite material 2 according to a preferred embodiment in thepresent invention, the ratio I_(D)/I_(G) (R value) of a peak strength(I_(D)) in the vicinity of 1360 cm⁻¹ to a peak strength (I_(G)) in thevicinity of 1580 cm⁻¹ as measured by Raman spectroscopy is preferably noless than 0.01 and no more than 0.2. When the R value is more than 0.2,a large number of highly activated edge portions of graphite materialare exposed to the surface thereof. Thus, during charging anddischarging, a large number of side reactions are more likely to occur.In contrast, when the R value is less than 0.01, barriers that preventthe entering and exiting of lithium are higher, and thus the currentload characteristic is more likely to be degraded.

The laser Raman R value is measured using “NRS3100” made by JASCOCorporation under the conditions that the excitation wavelength is 532nm, the width of the entrance slit is 200 μm, the exposure period is 15seconds, the totaling is performed twice and the diffraction grating has600 bars per millimeter.

In the graphite material 2 according to the preferred embodiment in thepresent invention, the coefficient of thermal expansion (CTE) attemperatures of 30 to 100° C. is preferably no less than 4.0×10⁻⁶/° C.and no more than 5.0×10⁻⁶/° C. The coefficient of thermal expansion isutilized as an indicator that shows the acicularity of coke. Althoughgraphite materials with a coefficient of thermal expansion of lower than4.0×10⁻⁶/° C. have high crystallinity of graphite and a high dischargecapacity, the particles thereof are more likely to form a plate-likeshape. In contrast, although graphite materials with a coefficient ofthermal expansion of higher than 5.0×10⁻⁶/° C. have a low aspect ratio,the crystals of graphite are not grown, and a discharge capacity is low.The CTE of the graphite material 2 is measured in the same manner asthat of a carbon raw material, which will be described later.

The graphite material 2 in the present invention has the averageinterplanar spacing d₀₀₂ of (002) plane as measured by X-ray diffractionof preferably 0.3362 nm to 0.3370 nm. The interplanar spacing d₀₀₂ canbe measured in a known manner by powder X-ray diffraction (XRD) (see the117th committee material 117-71-A-1 (1963) of Japan Society for thePromotion of Science by Inakichi Noda and Michio Inagaki, the 117thcommittee material 117-121-C-5 (1972) of Japan Society for the Promotionof Science by Michio Inagaki et al. and “carbon” No. 36, pp. 25-34, 1963by Michio Inagaki).

Preferably, in the graphite material 2 in the present invention, theloose bulk density is no less than 0.7 g/cm³, and the powder density isno less than 1.0 g/cm³ and no more than 1.35 g/cm³ after tapping isperformed 400 times.

When the graphite material 2 having the characteristics described aboveis used as carbon material for battery electrodes, high-energy densitybattery electrodes can be obtained while a high capacity, high coulombicefficiency and an excellent cycle characteristic are maintained.

(Graphite Material 3)

In a graphite material 3 according to a third embodiment in the presentinvention, D50% is 2 to 9 μm in particle diameter distribution based onvolume as measured by laser diffraction, the specific surface area is 2to 6 m²/g and no coating layer is substantially included in the surfaceof the particles. Or the graphite material 3 in the present inventioncomprises substantially single-composition particles having an isotropiccrystal structure, wherein D50% is 2 to 9 μm in particle diameterdistribution based on volume as measured by laser diffraction, thespecific surface area is 2 to 6 m²/g.

FIG. 1 shows a cross-sectional image of mesocarbon as observed bytransmission electron microscopy (TEM). As will be understood from FIG.1, in the mesocarbon, its surface layer A (coating layer) has a whitecolor (in a state where electron beams are more likely to betransmitted), and its inner layer B has a gray color (in a state whereelectron beams are less likely to be transmitted). A boundary C appearsclearly between the surface layer and the inner layer.

The cross-sectional image of the graphite material 3 in the presentinvention as observed by TEM has no boundary between the surface layerand the inner layer like that shown in FIG. 2, and has a gray colorevenly from the surface layer to the inner layer. That is, in thegraphite material in the present invention, no coating layer issubstantially included in the surface of the particles, or an isotropiccrystal structure is formed and a single composition is substantiallyformed.

In the graphite material 3 in the present invention, D50% is 2 to 9 μmin particle diameter distribution based on volume as measured by laserdiffraction.

The graphite material 3 in the present invention has the specificsurface area (as measured by BET method) of 2 to 6 m²/g. When thegraphite material has a specific surface area of more than 6 m²/g, arelatively large amount of PVDF-based binder or even SBR-based binderneeds to be added. This leads to the decreased mass of active materialper unit electrode volume, with the result that the battery capacity isdecreased. Since the specific surface area is increased, the surfaceactivity of the particles becomes higher, with the result that thecoulombic efficiency may be decreased due to decomposition ofelectrolytic solution or other factors.

In the graphite material 3 according to a preferred embodiment in thepresent invention, the ratio I_(D)/I_(G) (R value) of a peak strength(I_(D)) in the vicinity of 1360 cm⁻¹ to a peak strength (I_(G)) in thevicinity of 1580 cm⁻¹ as measured by Raman spectroscopy is preferably noless than 0.01 and no more than 0.2. When the R value is more than 0.2,a large number of highly activated edge portions of graphite materialare exposed to the surface thereof. Thus, during charging anddischarging, a large number of side reactions are more likely to occur.In contrast, when the R value is less than 0.01, barriers that preventthe entering and exiting of lithium are higher, and thus the currentload characteristic is more likely to be degraded.

The laser Raman R value is measured using “NRS3100” made by JASCOCorporation under the conditions that the excitation wavelength is 532nm, the width of the entrance slit is 200 μm, the exposure period is 15seconds, the totaling is performed twice and the diffraction grating has600 bars per millimeter.

The graphite material 3 in the present invention has the averageinterplanar spacing d₀₀₂ of (002) plane as measured by X-ray diffractionof preferably 0.3362 nm to 0.3370 nm. The interplanar spacing d₀₀₂ canbe measured in a known manner by powder X-ray diffraction (XRD) (see the117th committee material 117-71-A-1 (1963) of Japan Society for thePromotion of Science by Inakichi Noda and Michio Inagaki, the 117thcommittee material 117-121-C-5 (1972) of Japan Society for the Promotionof Science by Michio Inagaki et al. and “carbon” No. 36, pp. 25-34, 1963by Michio Inagaki).

In the graphite material 3 in the present invention, the aspect ratio(the maximum length D_(max)/the vertical length relative to the maximumlength DN_(max) (D_(max)=the maximum length between any two points onthe perimeter of the image of a particle; DN_(max)=the shortest lengthof a line that, when the image is interposed between two lines parallelto the line of the maximum length, vertically connects the two lines) ispreferably 1.0 to 1.32. The aspect ratio can be measured by imageanalysis using “FPIA3000” made by Sysmex Corporation. At least 3000points, preferably 30000 points or more, and more preferably 50000points or more are measured, and the average of the measured values isused.

Preferably, the graphite material 3 in the present invention has theloose bulk density of no less than 0.4 g/cm³, and the powder density ofno less than 0.5 g/cm³ and no more than 1 g/cm³ after tapping isperformed 400 times.

In the graphite material 3 according to the preferred embodiment in thepresent invention, the coefficient of thermal expansion (CTE) attemperatures of 30 to 100° C. is no less than 4.0×10⁻⁶/° C. and no morethan 5.0×10⁻⁶/° C. The coefficient of thermal expansion is utilized asan indicator that shows the acicularity of coke. Although graphitematerials with a coefficient of thermal expansion of lower than4.0×10⁻⁶/° C. have high crystallinity of graphite and a high dischargecapacity, the particles thereof are more likely to form a plate-likeshape. In contrast, although graphite materials with a coefficient ofthermal expansion of higher than 5.0×10⁻⁶/° C. have a low aspect ratio,the crystals of graphite are not grown, and a discharge capacity is low.The CTE of the graphite material 3 is measured in the same manner asthat of a carbon raw material, which will be described later.

When the graphite material having the characteristics described above isused as carbon material for battery electrodes, high-energy densitybattery electrodes can be obtained while a high capacity, high coulombicefficiency and an excellent cycle characteristic are maintained.

(Producing Method of Graphite Material)

The producing method of a graphite material in the present invention isnot particularly limited; the preferred producing method of a graphitematerial in the present invention includes steps where a carbon rawmaterial in which the loss on heat when the carbon raw material isheated from 300 to 1200° C. under an inert atmosphere is no less than 5%by mass and no more than 20% by mass is pulverized and then subjected toheat treatment at temperatures no less than 2000° C.

The carbon raw material used for the producing method in the presentinvention is such that the loss on heat when the carbon raw material isheated from 300 to 1200° C. under an inert atmosphere is no less than 5%by mass and no more than 20% by mass. When the loss on heat is less than5% by mass, particles are more likely to form a plate-like shape.Moreover, since the pulverized surface (edge portion) is exposed, thespecific surface area is increased. Thus, the number of side reactionsis increased. In contrast, when the loss on heat is more than 20% bymass, the attachment of graphitized particles is increased, and thisaffects efficiency. When the loss on heat falls within the rangedescribed above, the specific surface area of graphite material isreduced, and the number of side reactions is decreased. The reason whythe number of side reactions is decreased is not completely clear, butit is thought that the components volatilized by being heated attemperatures of 300 to 1200° C. are carbonized and graphitized, and thusthe crystal of the exposed edge portion is recrystallized andstabilized, and furthermore the surface of the particles is smoothed,and thus the specific surface area is reduced.

The loss on heat described above can be measured at an elevating rate of10° C./minute using a commercially available device that can perform TGand DTA. In the examples of the present invention and the like,measurements were performed using “TGDTAw6300” made by Seiko InstrumentsInc. according to the following procedure: about 15 mg of the sample tobe measured is accurately weighed, it is placed on a platinum pan and isthen set into the device, argon gas is passed at a rate of 200cm³/minute, the temperature is increased to 1400° C. at a rate of 10°C./minute and then the measurements are performed. As the reference,α-alumina made by Wako Pure Chemical Industries, Ltd. was used afterbeing treated at a temperature of 1500° C. for three hours for theremoval of volatile components.

The carbon raw material having the loss on heat as described above isselected from petroleum-derived pitch cokes or coal-derived pitch cokes.In particular, the carbon raw material used for the present invention ispreferably selected from green coke that is one of petroleum cokes. Thecrystal of green coke is not grown and thus is pulverized into aspherical shape, with the result that the specific surface area is morelikely to be reduced. Thus, the carbon raw material used for the presentinvention is preferably a non-needle coke, and more preferably anon-needle coke that is not subjected to heat treatment.

Petroleum cokes are black, porous, solid residues obtained by thecracking or cracking distillation of petroleum oil and bituminous oil.Petroleum cokes are divided into fluid cokes and delayed cokes accordingto the coking method. Fluid cokes, however, are powders with limitedapplications and are only used for refinery's home fuel, and thusdelayed cokes are commonly referred to as petroleum cokes. Delayed cokesare divided into green cokes (raw cokes) and calcined cokes. Green cokesare cokes without being further processed after being obtained from acoking device; calcined cokes are obtained by heating green cokes againfor the removal of volatile components. Since green cokes are likely tocause dust explosion, in order for fine-particle petroleum cokes to beobtained, green cokes are pulverized after being calcined for theremoval of volatile components. Conventionally, calcined cokes aregenerally used for electrodes and the like. Since green cokes have lessash content than coal cokes, they are only used for carbon materials inthe carbide industry, cokes for casting and cokes for alloy iron. Greencokes as described above are used in the present invention.

In the carbon raw material used in the present invention, thecoefficient of thermal expansion (CTE) at temperatures of 30 to 100° C.is preferably no less than 4.8×10⁻⁶/° C. and no more than 6.0×10⁻⁶/° C.The CTE of the carbon raw material can be measured as follows. By avibration mill, 500 g of the carbon raw material is pulverized so as tohave a size of 28 mesh or less. This sample is screened to obtain 60 gof the carbon raw material having a size of 28 to 60 mesh, 32 g of thecarbon raw material having a size of 60 to 200 mesh and 8 g of thecarbon raw material having a size of 200 mesh or less. They are mixed tohave the total weight of 100 g. This mixed sample is placed into astainless container, and 25 g of binder pitch is added to it. The sampleis heated and mixed evenly in an oil bath maintained at a temperature of125° C. for 20 minutes. This mixture is cooled and pulverized by thevibration mill so that all the mixture has a size of 28 mesh or less.Into a pressure molding machine maintained at a temperature of 125° C.,30 g of the sample is put, and is pressed and molded at a gauge pressureof 450 kg/cm² for five minutes. The molded sample is placed into aporcelain crucible. The temperature of the sample is increased in acalcining furnace from a room temperature to 1000° C. for five hours,the sample is maintained at a temperature of 1000° C. for one hour andis then cooled. This calcined sample is cut with a precision cuttingmachine to have a dimension of 4.3×4.3×20.0 mm to obtain the test piece.The thermal expansion measurement of this test piece at temperatures of30 to 100° C. is performed using TMA (thermo mechanical analyzer) suchas “TMA/SS 350” made by Seiko Instruments Inc., and consequently the CTEis determined.

Next, this carbon raw material is pulverized. The carbon raw material ispulverized with a known device such as a jet mill, a hammer mill, aroller mill, a pin mill or a vibration mill. Preferably, the carbon rawmaterial is so pulverized as to have as low a heat history as possible.If heat is applied by pulverizing, the amount of components volatilizedat temperatures of 300 to 1200° C. as described previously is decreased,and thus it is likely that the effects described previously cannot beobtained.

Preferably, the pulverized carbon raw material is classified to have anaverage particle size of 10 to 25 μM. When the average particle size islarge, the electrode density tends to be difficult to increase. Incontrast, when the average particle size is small, side reactions aremore likely to occur during charging and discharging. The particle sizeis measured by “CILUS” using laser diffractometry.

The pulverized carbon raw material may be calcined at a low temperatureof about 500 to 1200° C. before a graphitization process, which will bedescribed later. This low-temperature calcining process can reduce theamount of gas generated in the succeeding graphitization process. Thelow-temperature calcining process needs to be performed under anon-oxidizing atmosphere.

The pulverized carbon raw material is then graphitized. Thegraphitization process is preferably performed under an atmosphere inwhich the carbon raw material is difficult to oxidize. Examples of thegraphitization process include a heat treatment under an atmosphere ofargon gas or the like; and a heat treatment in an Acheson furnace(non-oxidation graphitization process). The non-oxidation graphitizationprocess is preferable in terms of cost.

The lower limit of the temperature of the graphitization process isgenerally 2000° C., preferably 2500° C., more preferably 2900° C., andmost preferably 3000° C. The upper limit of the temperature of thegraphitization process is not particularly limited, but is preferably3300° C. in terms of easily obtaining a high discharge capacity.

Preferably, in the process of the present invention, the graphitematerial is not broken into pieces nor pulverized after thegraphitization process. When the graphite material is broken into piecesor pulverized after the graphitization process, the smoothed surface maybe damaged, and this may result in degraded performance.

With this method, it is possible to obtain the graphite material havingthe structure as shown in FIG. 2.

(Carbon Material for Battery Electrodes)

The carbon material for battery electrodes according to the presentinvention comprises the graphite material of the present invention. Thecarbon material for battery electrodes is used as, for example, an anodeactive substance or an anode conductivity-enhancing agent of a lithiumsecondary battery.

The carbon material for battery electrodes according to the presentinvention further comprises a carbon fiber. With respect to 100 parts bymass of the graphite material, 0.01 to 20 parts by mass of the carbonfiber are preferably comprised.

Carbon fibers include organic carbon fibers such as PAN-based carbonfibers, pitch-based carbon fibers and rayon-based carbon fibers andvapor grown carbon fibers. The vapor grown carbon fibers are preferablesince they have high crystallinity and high thermal conductivity. Forexample, the vapor grown carbon fibers are produced according to thefollowing procedure: an organic compound is used as a raw material, theraw material and an organic transition metal compound serving as acatalyst are introduced with a carrier gas into a high-temperaturereaction furnace and then they are subjected to heat treatment (forexample, see JP-A-S60-054998 and Japanese Patent No. 2778434). Thediameter of the carbon fiber is preferably 2 to 1000 nm, and morepreferably 0.01 to 0.5 μm; the aspect ratio thereof is preferably 10 to15000.

The organic compounds serving as a raw material for the carbon fiberinclude toluene, benzene, naphthalene, gases such as ethylene,acetylene, ethane, natural gas and carbon monoxide and their mixtures.An aromatic hydrocarbon such as toluene or benzene is preferably used.

The organic transition metal compound includes a transition metalserving as a catalyst. Examples of the transition metal include metalsin IVa, Va, VIIa, VIIa and VIII groups in the periodic table. A compoundsuch as ferrocene or nickelocene is preferably used as the organictransition metal compound.

The carbon fiber used in the present invention may be obtained bybreaking into pieces or pulverizing long fibers produced by a vaporgrowth method or the like. The carbon fiber may be formed of flocs.

Preferably, the carbon fiber used in the present invention has nodecomposed substance derived from an organic compound or the likeattached to the surface thereof, or has high crystallinity of carbonstructure.

For example, the carbon fiber having no decomposed substance attached tothe surface thereof, or having high crystallinity of carbon structurecan be obtained by calcining (thermally treating) a carbon fiber orpreferably a vapor grown carbon fiber under an inert gas atmosphere.Specifically, the carbon fiber having no decomposed substance attachedto the surface thereof can be obtained by heat treatment under an inertgas atmosphere such as argon at temperatures of about 800 to 1500° C.The carbon fiber having high crystallinity of carbon structure can beobtained by heat treatment under an inert gas atmosphere such as argonat temperatures of preferably 2000° C. or higher, and more preferably2000 to 3000° C.

Preferably, the carbon fiber used in the present invention includesbranched fibers. The carbon fiber may partially have a continuous hollowstructure in the fibers. To have such a structure, carbon layersconstituting cylindrical portions of the fibers are continuous. Thehollow structure may partially have carbon layers wound like cylinders,and such carbon layers includes carbon layers wound like substantialcylinders, partially non-continuous carbon layers and two stacked carbonlayers combined into one layer. The shape of the cross sections of thecylinders is not limited to a complete circle, but may be oval orpolygonal.

In the favorable carbon fiber used in the present invention, the averageinterplanar spacing d₀₀₂ of (002) plane as measured by X-ray diffractionis preferably no more than 0.344 nm, more preferably no more than 0.339nm, and most preferably no more than 0.338 nm. The thickness (Lc) of acrystal in the direction of the C-axis is preferably no more than 40 nm.

(Paste for Electrodes)

The paste for electrodes according to the present invention comprisesthe above-described carbon material for battery electrodes and a binder.The paste for electrodes is obtained by kneading the carbon material forbattery electrodes and the binder. A known device such as a ribbonmixer, a screw kneader, a spartan lyser, a Lödige mixer, a planetarymixer or a universal mixer can be used for kneading process. The pastefor electrodes can be molded into a sheet, a pellet or other shapes.

Examples of the binder used in the paste for electrodes include thefollowing known binders: fluorinated binders such aspolyvinylidene-fluoride and polytetrafluoroethylene, and rubber-basedbinders such as SBR (styrene-butadiene rubber).

With respect to 100 parts by mass of the carbon material for batteryelectrodes, the amount of binder used is normally 1 to 30 parts by mass,and more preferably 3 to 20 parts by mass.

A solvent may be used for kneading process. Examples of the solventinclude the following known solvents suitable for binders: toluene,N-methylpyrrolidone and the like for fluorinated binders; water and thelike for rubber-based binders; and other solvents such asdimethylformamide and isopropanol. A thickener is preferably used with abinder using water as a solvent. The amount of solvent used is adjustedso that the viscosity of the binder is suitable in applying it to acollector.

(Electrode)

The electrode according to the present invention is formed of a moldedpiece of the above-described paste for electrodes. For example, theelectrode according to the present invention is obtained by applying thepaste for electrodes to a collector, drying it and pressure-molding it.

Examples of the collector include foils and meshes of aluminum, nickel,copper, stainless steel and the like. The thickness of the paste appliedis generally 50 to 200 μm. When the thickness of the paste applied istoo large, it is likely that the anode cannot be housed in astandardized battery case. The method of application of the paste is notparticularly limited. Examples of the applying method include thefollowing method: the past is applied with a doctor blade, a barcoateror the like, and it is then molded with a roll press or the like.

Methods of pressure-molding include roll-pressing and press-molding. Thepressure applied when the electrode is pressure-molded is preferablyabout 1 to 3 t/cm². As the electrode density of the electrode isincreased, the battery capacity per volume generally becomes higher.However, when the electrode density is excessively increased, the cyclecharacteristic is generally degraded. With the paste for electrodesaccording to the present invention, even when the electrode density isincreased, the cycle characteristic is less degraded. Thus, it ispossible to obtain electrodes having a high electrode density. Themaximum electrode density when the paste for electrodes according to thepresent invention is used is generally 1.7 to 1.9 g/cm³. The electrodethus obtained is suitable for the anode of a battery, and especially forthe anode of a secondary battery.

(Battery and Secondary Battery)

The battery or the secondary battery according to the present inventioncomprises the above-described electrode as a component (preferably as ananode).

A description will now be given of the battery and the secondary batteryaccording to the present invention by way of examples of a lithiumsecondary battery. The lithium secondary battery has a structure inwhich a cathode and an anode are immersed in electrolytic solution orelectrolyte. The electrode according to the present invention is used asthe anode.

As a cathode active substance used in the cathode of the lithiumsecondary battery, a lithium-containing transition metal oxide isgenerally used. Preferably, the lithium-containing transition metaloxide is an oxide that mainly contains at least one transition metalelement selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni,Mo and W, and lithium, the oxide being characterized in that the molarratio of lithium to the transition metal element is 0.3 to 2.2. Morepreferably, the lithium-containing transition metal oxide is an oxidethat mainly contains at least one transition metal element selected fromthe group consisting of V, Cr, Mn, Fe, Co and Ni, and lithium, the oxidebeing characterized in that the molar ratio of lithium to the transitionmetal element is 0.3 to 2.2. With respect to the mainly containedtransition metal, less than 30% by mol of Al, Ga, In, Ge, Sn, Pb, Sb,Bi, Si, P, B or the like may be contained. At least one substance havinga spinel structure represented by the general chemical formula “Li_(x)MO₂” (where M is at least one of Co, Ni, Fe and Mn, x=0 to 1.2) or thegeneral chemical formula “Li_(y) A₂ O₄” (where A contains at least Mn,y=0 to 2) is preferably used as the cathode active substance.

More preferably, a material containing a substance represented by thegeneral chemical formula “Li_(y) M_(a) D_(1-a) O₂” (where M is at leastone of Co, Ni, Fe and Mn, D is at least one of Co, Ni, Fe, Mn, Al, Zn,Cu, Mo, Ag, W, Ga, In, Sn, Pb, Sb, Sr, B and P other than M, y=0 to 1.2and a=0.5 to 1) or at least one substance having a spinel structurerepresented by the general chemical formula “Li_(Z) (A_(b) E_(1-b))₂ O₄”(where A is Mn, E is at least one of Co, Ni, Fe, Mn, Al, Zn, Cu, Mo, Ag,W. Ga, In, Sn, Pb, Sb, Sr, B and P, b=1 to 1.2 and z=0 to 2) is used asthe cathode active substance.

Specific examples of the cathode active substance include: Li_(x) Co O₂,Li_(x) Ni O₂, Li_(x) Mn O₂, Li_(x) Co_(a) Ni_(1-a) O₂, Li_(x) CO_(b)V_(1-b) O_(z), Li_(x) Co_(b) Fe_(1-b) O₂, Li_(x) Mn₂ O₄, Li_(x) Mn_(c)CO_(2-c) O₄, Li_(x) Mn_(c) Ni_(2-c) O₄, Li_(x) Mn_(c) V_(2-c) O₄ andLi_(x) Mn_(c) Fe_(2-c) O₄ (where x=0.02 to 1.2, a=0.1 to 0.9, b=0.8 to0.98, c=1.6 to 1.96 and z=2.01 to 2.3). Most preferably, examples of thelithium-containing transition metal oxide include: Li_(x) Co O₂, Li_(x)Ni O₂, Li_(x) Mn O₂, Li_(x) Co_(a) Ni_(1-a) O₂, Li_(x) Mn₂ O₄ and Li_(x)Co_(b) V_(1-b) O_(z) (where x=0.02 to 1.2, a=0.1 to 0.9, b=0.9 to 0.98,z=2.01 to 2.3). The “x” represents a value before the start of chargingand discharging. The value “x” increases or decreases depending oncharging and discharging.

The average particle size of the cathode active substance is notparticularly limited, but is preferably 0.1 to 50 μm. Preferably, thevolume of particles having a diameter of 0.5 to 30 μm accounts for 95%or more. More preferably, the volume of particles having a diameter of 3μm or less accounts for 18% or less of the entire volume, and the volumeof particles having a diameter no less than 15 μm and no more than 25 μmaccounts for 18% or less of the entire volume. The specific surface areais not particularly limited. The specific surface area measured by BET,however, is preferably 0.01 to 50 m²/g, and more preferably 0.2 to 1m²/g. The pH of the supernatant portion of the solution in which 5 g ofthe cathode active substance is dissolved in 100 ml of distilled wateris preferably no less than 7 and no more than 12.

In the lithium secondary battery, a separator may be provided betweenthe anode and cathode. Examples of the separator include a nonwovenfabric, a cloth, a microporous film and their combinations that have, asa main ingredient, polyolefin such as polyethylene or polypropylene.

As the electrolytic solution and electrolyte comprised in the lithiumsecondary battery according to the present invention, known organicelectrolytic solution, inorganic solid electrolyte or polymeric solidelectrolyte may be used. The organic electrolytic solution is preferablyused in terms of electrical conductivity.

The organic solution including one of the following substances ispreferably used as the organic electrolytic solution: ethers such asdiethyl ether, dibutyl ether, ethylene glycol monomethyl ether, ethyleneglycol monoethyl ether, ethylene glycol monobutyl ether, diethyleneglycol monomethyl ether, diethylene glycol monoethyl ether, diethyleneglycol monobuthyl ether, diethylene glycol dimethyl ether or ethyleneglycol phenyl ether; amides such as formamide, N-methyl formamide,N,N-dimethyl formamide, N-ethyl formamide, N,N-diethyl formamide,N-methyl acetamide, N, N-dimethyl acetamide, N-ethyl acetamide,N,N-diethyl acetamide, N,N-dimethyl propion amide or hexamethylphosphoryl amide; sulfuric compounds such as dimethyl sulfoxide orsulfolane; dialkyl ketones such as methyl ethyl ketone or methylisobutyl ketone; cyclic ethers such as ethylene oxide, propylene oxide,tetrahydrofuran, 2-methoxy tetrahydrofuran, 1,2-dimethoxyethane or1,3-dioxolan; carbonates such as ethylene carbonate or propylenecarbonate; γ-butyrolactone; N-methylpyrrolidone; acetonitrile; ornitromethane. More preferably, the solution including one of thefollowing substances is used as the organic electrolytic solution:esters such as ethylene carbonate, butylene carbonate, diethylcarbonate, dimethyl carbonate, propylene carbonate, vinylene carbonateor γ-butyrolactone; ethers such as dioxolan, diethyl ether ordiethoxyethane; dimethyl sulfoxide; acetonitrile; tetrahydrofuran; orthe like. Most preferably, the solution including one of the followingsubstances is used as the organic electrolytic solution: carbonate-basednonaqueous solvent such as ethylene carbonate or propylene carbonate. Ofthese solvents, only one solvent or a combination of two or moresolvents may be used.

Lithium salt is used as a solute (electrolyte) for these solvents.Common lithium salts include LiClO₄, LiBF₄, LiPF₆, LiAlCl₄, LiSbF₆,LiSCN, LiCl, LiCF₃SO₃, LiCF₃CO₂ and LiN(CF₃SO₂)₂.

Examples of the polymeric solid electrolyte include a polyethylene oxidederivative, a polymer containing the polyethylene oxide derivative, apolypropylene oxide derivative, a polymer containing the polypropyleneoxide derivative, a phosphoric acid ester polymer, a polycarbonatederivative, and a polymer containing the polycarbonate derivative.

The selection of members necessary for the battery other than thosedescribed above is not limited at all.

Since the battery or the secondary battery according to the presentinvention has an excellent charge-discharge cycle characteristic and anexcellent large-current load characteristic, it can be applied to thefields of, for example, electric tools such as an electric drill andhybrid cars and the like, where conventionally, lead-acid secondarybatteries, nickel-cadmium secondary batteries and nickel-metal-hydridesecondary batteries are mainly used.

EXAMPLES

Hereinafter, a more specific description will be given by way of atypical example of the present invention. It should be noted that thefollowing description is illustrative only and the present invention isnot limited to the following description.

The properties or the like described in examples below were measuredaccording to the following methods.

(Specific Surface Area)

The specific surface areas were measured using a specific-surface-areameasuring device “NOVA-1200” (made by Yuasa Ionics Inc.) by the commonlyused BET method.

(Battery Evaluation Method)

(1) Preparation of the Paste

0.1 part by mass of “KF polymer L1320” (an N-methylpyrrolidone (NMP)solution containing 12% by mass of polyvinylidene-fluoride (PVDF)) madeby Kureha Corporation were added to 1 part by mass of graphite material,and an undiluted solution of a base compound was obtained by kneadingthem with a planetary mixer.

(2) Fabrication of the Electrode

NMP was added to the undiluted solution of the base compound, theirviscosity was thereby adjusted, and they were then applied tohigh-purity cooper foil using a doctor blade so as to have a thicknessof 250 μm. This was vacuum-dried at a temperature of 120° C. for onehour, and then was press-cut so as to have a diameter of 18 mm. Thepress-cut electrode was sandwiched and pressed by a super-steel pressplate at a pressure of about 1×10² to 3×10² N/mm² (1×10³ to 3×10³kg/cm²). Thereafter, it was vacuum-dried at a temperature of 120° C. for12 hours and was used as an electrode for evaluation.

(3) Fabrication of the Battery

A three-pole cell was made as follow. The following process wasperformed under the atmosphere of dry argon having a dew point of −80°C. or less.

In a cell (having an inside diameter of about 18 mm) of polypropylenewith a screw lid, the carbon electrode attached to the cooper foil madeaccording to process (2) described above and metal lithium foil weresandwiched and laminated with a separator (a microporous film ofpolypropylene (“cellguard 2400”)). Similarly, metal lithium forreference was laminated. An electrolytic solution was added to this, sothat a testing cell was obtained.

(4) Electrolytic Solution

LiPF₆ (1 mol/liter) was dissolved as an electrolyte in a liquid mixturecomposed of 8 parts by mass of EC (ethylene carbonate) and 12 parts bymass of DEC (diethyl carbonate).

(5) Charge-Discharge Cycle Test

A constant current and constant voltage charge-discharge test wasperformed at a current density of 0.2 mA/cm² (corresponding to 0.1 C).

Constant-current charging (intercalation of lithium into carbon) wasperformed at a current density of 0.2 mA/cm² such that the voltage ischanged from a rest potential to 0.002 V. Then, when the voltage reached0.002 V, the constant-current charging was switched to constant-voltagecharging, and when the current was decreased to 25.4 μA, theconstant-voltage charging was stopped.

Constant-current discharging (emission of lithium from carbon) wasperformed at a current density of 0.2 mA/cm² (corresponding to 0.1 C)and was cut off at a voltage of 1.5V.

(Graphite Material 1)

Example 1

Petroleum derived green coke (non-needle coke) in which the loss onheat, as measured by TG, when it is heated from 300 to 1200° C. was11.8% by mass was pulverized using a bantam mill made by Hosokawa MicronCorporation. The pulverized petroleum derived green coke was classifiedby air current using “Turbo Classifier” made by Nisshin EngineeringInc., and thus a carbon raw material was obtained that had a D50 of 14.2μm. The pulverized carbon raw material was loaded into a graphitecrucible with a screw lid, and was graphitized in an Acheson furnace ata temperature of 3000° C. Consequently, the graphite material wasobtained that was characterized in that the laser Raman R value was 0.03and the CTE was 4.2×10⁻⁶° C.⁻¹. Since the graphite material thusobtained had a small specific surface area, a battery was obtained thathad an excellent discharge capacity, excellent beginning efficiency andan excellent cycle characteristic. The result is shown in Table 1.

TABLE 1 Ex. 1 Ex. 2 Comp. Ex. 1 Comp. Ex. 2 Production process Greencoke Green coke Green coke Green coke (non-needle) → (non-needle) →(non-needle) → (non-needle) → pulverizing → pulverizing → calcininggraphitization → graphitization calcining → 1200° C. → pulverizinggraphitization pulverizing → graphitization Carbon raw materialPetroleum Petroleum Petroleum Petroleum coke coke coke coke loss on heat11.8 12.1 1.2 1.3 at 300° C.-1200° C.[%] History maximum- 400 400 12003000 temperature before pulverizing [° C.] Graphitization process 30003000 3000 3000 temperature [° C.] Graphite material properties LaserRaman 0.03 0.03 0.06 0.50 R value CTE [° C.⁻¹] 4.2 × 10⁻⁶ 4.3 × 10⁻⁶ 4.1× 10⁻⁶ 3.8 × 10⁻⁶ d₀₀₂ [nm] 0.3365 0.3364 0.3366 0.3366 Average particle14.2 14.8 14.9 15.2 diameter D50 [μm] Specific surface 1.41 1.39 2.105.50 area [m²/g] Tap density (400 times) 1.28 1.29 1.15 0.97 [g/cm³]Battery characteristics Discharge capacity 332 330 332 335 [mAh/g]Beginning efficiency [%] 94.2 94.3 93.8 90.0

Example 2

The same test as in Example 1 was performed except that after thepulverizing had been completed, a heat treatment at a temperature of1200° C. (low-temperature calcining) was performed beforegraphitization. The result is shown in Table 1.

Comparative Example 1

The same test as in Example 1 was performed except that before beingpulverized, the green coke was subjected to a heat treatment at atemperature of 1200° C. (calcining). The result is shown in Table 1. Thespecific surface area was large, and the tap density was slightly low.

Comparative Example 2

The green coke was graphitized at a temperature of 3000° C. withoutbeing pulverized and was then pulverized and classified by air currentas in Example 1. The same analyses and battery evaluation as in Example1 were performed. The result is shown in Table 1. The specific surfacearea was extremely large, and the beginning efficiency was decreased.

Table 1 shows that the graphite materials (in comparative examples) thatwere heated by the calcining and graphitizing processes to have a losson heat of less than 5% by mass and that were then pulverized provide alow discharge capacity and poor beginning efficiency. Table 1 also showsthe following. The carbon raw materials in which the loss on heat whenthe carbon materials were heated from 300 to 1200° C. under an inertatmosphere was no less than 5% by mass and no more than 20% by mass werepulverized, and the pulverized carbon raw materials were thengraphitized, with the result that carbon materials (in examples) wereobtained in which the ratio I_(D) / I_(G) (R value) of a peak strength(I_(D)) in the vicinity of 1360 cm⁻¹ to a peak strength (I_(G)) in thevicinity of 1580 cm⁻¹ as measured by Raman spectroscopy was no less than0.01 and no more than 0.2, and the coefficient of thermal expansion(CTE) at temperatures of 30 to 100° C. was no less than 4.0×10⁻⁶/° C.and no more than 5.0×10⁻⁶/° C. Table 1 further shows that the use of thegraphite materials of the examples as electrode active materials resultsin a high discharge capacity and an excellent beginning efficiency.

(Graphite Material 2)

Example 3

Petroleum derived green coke in which the loss on heat, as measured byTG, when it was heated from 300 to 1200° C. was 11.8% by mass waspulverized using a bantam mill made by Hosokawa Micron Corporation. Thepulverized petroleum derived green coke was classified by air currentusing “Turbo Classifier” made by Nisshin Engineering Inc., and thus acarbon raw material was obtained that had a D50 of 14.2 μm. Thepulverized carbon raw material was loaded into a graphite crucible witha screw lid, and was graphitized in an Acheson furnace at a temperatureof 3000° C. Consequently, the graphite material was obtained that wascharacterized in that the laser Raman R value was 0.03 and the CTE was4.2×10⁻⁶° C.⁻¹. Since the graphite material thus obtained had a smallspecific surface area, a battery was obtained that had an excellentdischarge capacity, excellent beginning efficiency and an excellentcycle characteristic. The result is shown in Table 2.

TABLE 2 Ex. 3 Ex. 4 Comp. Ex. 3 Comp. Ex. 4 Production process Greencoke Green coke Green coke Green coke (non-needle) → (non-needle) →(non-needle) → (non-needle) → pulverizing → pulverizing → calcininggraphitization → graphitization calcining → 1200° C. → pulverizinggraphitization pulverizing → graphitization Carbon raw materialPetroleum Petroleum Petroleum Petroleum coke coke coke coke loss on heat11.8 12.1 1.2 1.3 at 300° C.-1200° C.[%] History maximum- 400 400 12003000 temperature before pulverizing [° C.] Graphitization process 30003000 3000 3000 temperature [° C.] Graphite material properties Aspectratio 1.31 1.30 1.35 1.37 Laser Raman 0.03 0.03 0.06 0.50 R value CTE [°C.⁻¹] 4.2 × 10⁻⁶ 4.3 × 10⁻⁶ 4.1 × 10⁻⁶ 3.8 × 10⁻⁶ d₀₀₂ [nm] 0.33650.3364 0.3366 0.3366 Average particle 14.2 14.8 14.9 15.2 diameter D50[μm] Specific surface 1.41 1.39 2.10 5.50 area [m²/g] Loose bulk 0.920.93 0.81 0.68 density [g/cm³] Tap density (400 times) 1.28 1.29 1.150.97 [g/cm³] Battery characteristics Discharge capacity 332 330 332 335[mAh/g] Beginning efficiency [%] 94.2 94.3 93.8 90.0

Example 4

The same test as in Example 3 was performed except that after thepulverizing had been completed, a heat treatment at a temperature of1200° C. (low-temperature calcining) was performed beforegraphitization. The result is shown in Table 2.

Comparative Example 3

The same test as in Example 3 was performed except that before beingpulverized, the green coke was subjected to a heat treatment at atemperature of 1200° C. (calcining). The result is shown in Table 2. Thespecific surface area was large, and the tap density was slightly low.

Comparative Example 4

The green coke was graphitized at a temperature of 3000° C. withoutbeing pulverized, and was then pulverized and classified by air currentas in Example 3. The same analyses and battery evaluation as in Example3 were performed. The result is shown in Table 2. The specific surfacearea was extremely large, and the beginning efficiency was decreased.

(Graphite Material 3)

Example 5

Petroleum derived green coke in which the loss on heat, as measured byTG, when it was heated from 300 to 1200° C. was 11.8% by mass waspulverized using a bantam mill made by Hosokawa Micron Corporation. Thepulverized petroleum derived green coke was classified by air currentusing “Turbo Classifier” made by Nisshin Engineering Inc., and thus acarbon raw material was obtained that had a D50 of 4.8 μm. Thepulverized carbon raw material was loaded into a graphite crucible witha screw lid, and was graphitized in an Acheson furnace at a temperatureof 3000° C. Consequently, the graphite material was obtained that wascharacterized in that the laser Raman R value was 0.03 and the CTE was4.2×10⁻⁶° C.⁻¹. Since the graphite material thus obtained had a smallspecific surface area, a battery was obtained that had an excellentdischarge capacity, excellent beginning efficiency and an excellentcycle characteristic. The result is shown in Table 3.

TABLE 3 Comp. Ex. 5 Ex. 6 Green coke Ex. 5 Green coke (non-needle) →Green coke (non-needle) → calcining (non-needle) → pulverizing → 1200°C. → pulverizing → calcining → pulverizing → Production processgraphitization graphitization graphitization Carbon raw materialPetroleum Petroleum Petroleum coke coke coke loss on heat 11.8 12.1 1.2at 300° C.-1200° C. [%] History maximum- 400 400 1200 temperature beforepulverizing [° C.] Graphitization 3000 3000 3000 process temperature [°C.] Graphite material properties Aspect ratio 1.31 1.30 1.35 Laser RamanR value 0.05 0.04 0.06 CTE [° C.⁻¹] 4.2 × 10⁻⁶ 4.3 × 10⁻⁶ 4.1 × 10⁻⁶d₀₀₂ [nm] 0.3365 0.3364 0.3366 Particle D10 1.5 1.3 1.2 diameter D50 4.54.4 4.2 distribution D90 8.3 8.5 8.2 [μm] Specific surface 3.30 3.4013.20 area [m²/g] Loose bulk 0.52 0.55 0.19 density [g/cm³] Tap density0.63 0.67 0.22 (400 times) [g/cm³] Battery characteristics Dischargecapacity 332 330 332 [mAh/g] Beginnig efficiency 90.2 90.3 85.5 [%]

Example 6

The same test as in Example 5 was performed except that after thepulverizing had been completed, a heat treatment at a temperature of1200° C. (low-temperature calcining) was performed beforegraphitization. The result is shown in Table 3.

Comparative Example 5

The same test as in Example 5 was performed except that before beingpulverized, the green coke was subjected to a heat treatment at atemperature of 1200° C. (calcining). The result is shown in Table 3. Thespecific surface area was large, and the tap density was slightly low.

The invention claimed is:
 1. A method for producing a graphite material,comprising the steps of: pulverizing a carbon raw material which is araw non-needle coke, which raw non-needle coke is no less than 5% massand no more than 20% by mass in loss on heat when the carbon rawmaterial is heated from 300 to 1200° C. under an inert atmosphere; andthen graphitizing the pulverized carbon raw material which has anaverage particle size of not more than 25 μm, so as to obtain a graphitematerial having a CTE at temperatures of 30 to 100° C. of no less than4.0×10⁻⁶/° C. and no more than 5.0×10⁻⁶/° C.
 2. The method for producinga graphite material according to claim 1, wherein the carbon rawmaterial is derived from petroleum or coal.
 3. The method for producinga graphite material according to claim 1, wherein the graphitizationtemperature is no less than 3000° C. and no more than 3300° C.
 4. Themethod for producing a graphite material according to claim 1, whereinthe graphitization is performed in an Acheson furnace.
 5. The method forproducing a graphite material according to claim 1, in which the carbonraw material is no less than 4.8×10⁻⁶/° C. and no more than 6.0×10⁻⁶/°C. in a coefficient of thermal expansion (CTE) at temperatures of 30° C.to 100° C.
 6. The method for producing a graphite material according toclaim 1, further comprising the step of calcining the pulverized carbonraw material at 500° C. to 1200° C. before the graphitizing step.
 7. Themethod for producing a graphite material according to claim 1, in whichthe maximum temperature in the history of the carbon raw material beforethe pulverizing step is not more than 400° C.
 8. The method forproducing a graphite material according to claim 1, in which thepulverized carbon raw material has an average particle size of 10 to 25μm.